Add more sponsors to footer

[kcopas]

Could we add two more sponsors to the footer?

• NLBIF
• UN Environment

Source file for UN Environment is already white-on-transparent—just requires whatever resizing you want/need:

Waiting to see if NLBIF has a reversed out version of this:

[kcopas]

Two more additions for the footer:

• GBIF [linking to, well, you know :-)
• NLBIF [linking to https://www.nlbif.nl]

Source files located here: https://gbif.box.com/s/zgk5lp7lsm6oc2mw61txryr67f4s24j0

[MattBlissett]

@MortenHofft, some minor CSS change might be helpful for mobile device display of these logos.